A pump jack is the above ground drive for a reciprocating piston pump in a well. It is used to mechanically lift liquid, such as oil, out of the well if there is not enough bottom hole or formation pressure for forcing the liquid to flow up to the surface. Pump jacks are commonly used for onshore wells. A pump jack converts the rotary mechanism of a drive motor to a vertical reciprocating motion to drive the pump shaft, and displays a characteristic nodding motion.
Modern pump jacks are powered by a prime mover, which commonly comprises an electric motor. The prime mover runs a set of pulleys that, in turn, drive a pair of cranks, generally fitted with counterweights to assist the motor in lifting the heavy string of the rod line running into the ground. The cranks raise and lower one end of a beam, which is free to move on an A-shaped frame. On the other end of the beam is a “donkey head”, so named due to its appearance. The donkey head moves up and down as the cranks rotate.
An induction or asynchronous motor is an alternating current (“AC”) motor in which all electromagnetic energy is transferred by inductive coupling from a primary winding to a secondary winding, the two windings separated by an air gap. In both induction and synchronous motors, the AC power supplied to a stator disposed in the motor creates a magnetic field that rotates in time with the frequency of the AC power. A synchronous motor's rotor turns at the same rate as the stator field. In contrast, an induction motor's rotor rotates at a slower speed than the stator field. The induction motor stator's magnetic field is, therefore, changing or rotating relative to the rotor. This induces an opposing current in the induction motor's rotor, in effect, the motor's secondary winding when the latter is short-circuited or closed through an external impedance. The rotating magnetic flux induces currents in the rotor windings in a manner similar to currents induced in a transformer's secondary windings. These currents, in turn, create magnetic fields in the rotor that react against the stator field. Due to Lenz's Law, the direction of the magnetic field created will be such as to oppose the change in current through the windings. The cause of the induced current in the rotor windings is the rotating stator magnetic field, so to oppose this effect the rotor will start to rotate in the direction of the rotating stator magnetic field. The rotor accelerates until the magnitude of the induced rotor winding current and torque balances the applied load. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slower than synchronous speed.
For the motor to run, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (ηs), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field, as seen by the rotor (slip speed), and the rotation rate of the stator's rotating field is called “slip”. Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as an induction generator by running the motor at a speed higher than the synchronous speed of the stator magnetic field. In other words, by running the motor a negative slip.
Slip, s, is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in revolutions per minute (“RPM”), or in percent or ration of synchronous speed. Thus:
  s  =            η              s        -                  η          r                            η      s      where ηs is the synchronous speed of the stator magnetic field; and ηr is the rotor mechanical speed.
Therefore, as the motor operates to lift the donkey head, the motor consumes electrical power from an electrical power grid. In doing so, potential energy is created in lifting the donkey head. As the donkey head falls, the potential energy can be converted to kinetic energy as the motor can operate as a generator to generate electricity to put back onto the electrical power grid.
Underwriters Laboratories standard no. UL1741 is an accepted standard for grid interconnection with an electrical utility for inverter-based micro-generation technology, such as used in wind-generated electricity technology.
It is also known to use induction motors in some applications, such as operating a crane or elevator lifts, as a generator to put electricity back onto an electrical grid, but there are no applications using pump jack motors to do the same.
It is, therefore, desirable to provide a pump jack controller to harness the potential energy generated in operating a pump jack and convert that potential energy into electricity that can be put back onto an electrical grid in compliance with standards for micro-generation equipment.